1. Technical Field
This invention relates to gain cells and, more particularly, to gain cells with internal loss-compensation which can be employed in distributed amplifiers and other circuits.
2. Discussion
In practice, three terminal semiconductor devices such as silicon bipolar junction transistors (BJTs), metal-oxide semiconductor field effect transistors (MOSFETs), and their gallium arsenide (GaAs) and indium phosphide (InP)-based counterparts, hetero-junction bipolar technology (HBTs), metal semiconductor field effect transistor (MESFETs), and high electron mobility transistors (HEMTs), have significant performance limitations when used as gain cells in distributed amplifiers and other circuits due to intrinsic properties thereof. These types of devices have finite series and/or shunt resistances associated with each of their terminals which attenuate the power of an input signal delivered to the intrinsic device or limit the power delivered from the device to a load. In general, these device resistive losses can significantly limit the maximum gain and output power available from the device when used as a gain cell.
A distributed amplifier typically includes a signal input, a signal output, and two or more gain cells each having an input connected to the signal input and an output connected to the signal output. Preferably, each additional gain cell increases the gain and/or bandwidth of the distributed amplifier incrementally. The distributed amplifier is of particular interest because its fundamental gain-bandwidth performance limit is determined by the properties of the gain cell employed. Two main device characteristics that limit the gain-bandwidth performance of the distributed amplifier: 1) high input capacitance, and 2) the resistive losses of the input and output of a common-source/emitter configured device. The first limitation can be circumvented using capacitive coupling techniques, scaling the device size and/or choosing an optimum device quiescent bias, or intrinsic device enhancements. But for a given device size, bias and effective input capacitance, the device resistances and losses associated therewith cannot practically be reduced because they are a result of the fundamental limitation of the device and fabrication technology. The resistive losses of the input and output transmission lines of the distributed amplifier, due to these parasitic and intrinsic device characteristics, limit the number of stages which can ultimately be added to achieve increased amplifier output power. At some point, adding additional stages degrades the bandwidth without increasing the gain and output power. Therefore, a gain cell which has lower input capacitance and compensation for resistive losses is desirable in distributed amplifiers. Furthermore, a distributed amplifier employing gain cells with compensated resistive losses to allow an increased number of stages to be added without degradation of gain and bandwidth performance is also desirable.
Because of the fundamental differences in bipolar and field effect transistor device physics and fabrication technology, bipolar devices, and in particular, HBT devices, will suffer from larger resistive attenuation degradation in distributed amplifiers due to their generally more resistively lossy device input characteristics. Thus, it would be particularly desirable to provide enhanced gain, bandwidth, and output power of distributed amplifiers using BJTs, and in particular, HBTs.
Therefore, a gain cell using bipolar transistors which has lower input capacitance and compensation for resistive losses is desirable. Further, a distributed amplifier employing bipolar gain cells with compensated resistive losses to allow an increased number of stages to be added without degradation of gain and bandwidth performance is also desirable.